Wiring Ejection Charges

Why were all of the years odd???

David Erbas-White

Please think very hard before you use a key switch. They require keys to operate, right? What happens if the rocket lands far enough away (or is lost) and someone else finds it before you get to it with the key?

What happens if there is a live charge left in the rocket?

Use something that is easily operated with common tools (a screwdriver, for example). Label it, so that e.g., if your rocket is hanging from a powerline, the lineman can disarm it before fooling with it.

Also, make sure the switch you're using is both rated for the current it will be drawing, and mechanically able to withstand the vibration it will encounter. A surplus mil std microswitch, securely mounted, and operated by a bolt inserted into the airframe, can work really well.

Standard practice is to switch the positive wire, isn't it?

--tc

Read the manual that comes with whatever electronic device you buy. You might also read the manuals of the devices you are interested in before you buy one so you have an idea of how the various devices work.

Ask people at the launch. Most people are happy to answer questions and give advice.

Phil

Does one that happend on one of the LDRS shows that aired on the Discovery CHannel a few years count? THere are plenty of others.

Wasn't something changed here this spring for ALL electronic deployment flights in the HPR safety code revision as a result of the Safety Committee.

NSL Muncie, a BP charge went off in someones prep area.

What increased the danger of an ematch is not its connection to an altimeter, but it's proximity to a BP charge. Once in a container with several grams of BP, you have to treat them VERY differently.

RF in and of it's self can not set off an ematch... What can possibly happen: Extreme RF can cause transitory voltage to develop in some electronic circuits and the transitory voltage can then cause triggering of a firing circuit that would then in turn, fire the ematch. Although that is in theory, I have never known a case of this happening, with any of the devices we use. On the other hand, there is several altimeters that have or, allegedly have, a history of firing because of "switch bounce". Because of that problem I don't skimp on switches. I use either a parallel wired DPDT slide switch or a screw type arming switch to power up or down, all altimeters, Missleworks, Transolve, PerfectFlite, and yes even Adept. I sometimes use shunts with bigger charges because it makes me feel better, no other logical reason. I also cut the shunts before powering-up the altimeter unit as I don't like the idea of the unit circuit first reading 0 resistance, (dead short), and then + resistance from the ematch if I cut the shunt after power-up. Once again, no real reason, other than this "does seem logical".(:-)

Fred

A couple years ago I was prepping for a flight. I had recently added a switch inside my ebay to turn on or off the Perfectflight MAWD altimeter. I was testing the altimeter power switch. The events went like so...

I turned the switch on. (Insert screw). The Altimeter started beeping away indicating all good and there was continuity to the ematches. Then I turned the altimeter back off. (Remove screw). The altimeter didn't turn off. I used the wrong screw and the switch got mashed and therefore was jammed in the on position. Now I have a rocket that has charges armed and ready to go off. Well I have a couple other friends and flyers next to me as I'm doing this and after trying to bump the switch back to the off position I finally decide the only way I can get the power off now is to disassemble the rockets electronics bay and remove the power source or power lead from the battery. As I go to pull apart the rocket I unknowingly put my hand over the altimeter vent. I must say I and my friends are all really glad I do a lot of ground testing and fine tune my ejection charges rather than using the blow it out or blow it up idea because as I closed my hand over the port, POP. The drogue charge goes off. The bottom of the 12 pound rocket goes slamming into the ground at my feet. Now I'm a little dazed as to why that just happened but I figured it out as I was setting the rest of the rocket down and I take my hand off the vent hole. POP! the main charge goes off this time. The nose cone narrowly misses my friend!

I learned several things that day.

#1 Screw switches should be removed to arm not installed to arm. #2 All ematches should be shorted until the rocket is on the pad and ready for the igniter. #3 Never cover a barometric vent hole #4 Keep the pointy end up

I changed my set-up after that incident. Now I have a new set up using switches that will short up to 4 ematches individually. As long as the screws are through the airframe holding down the switch the altimeter will not get continuity and the ematches are shorted so they are safe from static and RF energy. I also use a separate connection or switch to arm the electronics.

Layne Rossi

Humm,, thinking on this one.

Shunting could damage the FET on an altimeter if not done right. then the altimeter will act like it works, but never fire the charge, resulting in a balistic recovery.

Almost all incidents I have seen of many charges going off wrong, shunts would not have helped.

They either would fire the charge when the shunt was pulled, or fried the altimeters FET and resulted in a ballistic recovery instead.

now we need to determine what's worse, a charge going off or a ballistic recovery ?

Cranny Dane

yes it seems to be.

but electrons flow from negative to positive, so if you switch the negative, you shut off the electrons before they leave the battery ?

Why were you arming the charges before the rocket was on the pad ?

If you wanted to check ebay setup with the ematches, do that before you you ever add BP.

Once you ad BP, don't turn on or arm anything until you get the rocket mounted on the pad.

I'm not sure that was the right lesson. The screw switches are a great idea, but they have to be engineered properly, just like everything else.

If your rocket floats away, or lands on a powerline, how is someone going to disarm it? Even if you label the holes "Insert 4-40 x 2" screw here" who is going to have one of those?

Better to figure out a way to keep the switch from jamming, or only carry one size of screw, or color-code the screws, than to have a rocket that needs to be disarmed and have someone not know how to do it!

--tc

No I have not checked out Marks Modern High Power Rocketry 2. I did not know about it. But thanks to you I can now. I hope you have a good launch this am. I won't be there. I am a church goer so thats where I'll be sitting. Have fun though. See you at the next saturday launch.

Thanks Kimball

TO clarify, I was not saying that incident was caused by RF. Neither do I believe that Woody was asking about accidental discharge being caused by RF.

Phil

THe FET will not be damaged by shunting under NORMAL circumstances. IF the outputs of the FET are shunted AND it attempts to fire the charge they can be but they are not certain to be. So the potential FET damage may possibly occur under conditions where you are trying to prevent charges froom going off / trying to insure safety. Seems like worthwhile to me.

Phil

Ted is correct. Switching the positive is standard in electronics. Electrons don't flow unless there is something (like the positive lead) to attract them.

Phil

Humm..

I've heard removing the negative (switching it) will help black wire disease ?

CD

Sorry, but "never" seems to be a strong term.

At the time I was looking into my L3, it was a requirement. And to me, one that was foolish. I did my l3 with TRA. This was back when Lubliner (the head of the L3CC) was still trying to get his own L3 cert.

I was there and know the history.

The discussion was about RF causing an ematch to "go off unexpectedly". Yes, I've seen ematches go off unexpectedly, but ALL were caused by flier errors. Your LDRS example just proves the case. A shunt or open would not have changed what happened at LDRS, as the rocket was being armed at the time

There was a bozo in my club that wanted to check out his electronics, and he took his rocket apart to ask questions and then put it back together. "Proof", or more correctly "Boom", as he didn't use a power switch, and the change in pressure triggered the ejection charge, withing a few feet of other folks.

I was asking about RF and ematches.. Without a doubt, as that's what "shunting" was to prevent.

There's nothing that will prevent a flier from screwing up his/her wiring and having an ematch trigger at the wrong time, including shunts/opens.